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Postoperative cognitive dysfunction (POCD) is an important complication of major surgery that predisposes to an increased risk of perioperative mortality and premature unemployment [1–5]. Although first recognized following cardiac surgery, in which 20–40% of patients are affected, long-term POCD also occurs after noncardiac surgery affecting more than 10% of patients greater than 60 years old [2]. In cardiac surgery, hypoperfusion, cerebral embolism, atrial fibrillation, previous cerebral injury or impairment of cognition as well as the systemic inflammatory response have been implicated in the development of POCD [5]. Risk factors for noncardiac surgical POCD include increasing age, duration of anaesthesia, previous cerebrovascular injury [3], paucity of higher education, a second operation, postoperative respiratory complications and infections [2]. Despite differing cited causes, both forms of POCD involve surgical stimulation, with incipient neuroinflammatory consequences [6], in an elderly patient; known risk factors such as cerebrovascular injury [3] or further operations and infective complications [2] could predispose to, or exacerbate, a cognitive vulnerability to neuroinflammation. In a previously published issue of the European Journal of Anaesthesiology, Zhu et al. [7] provided fascinating preliminary evidence that myocardial ischaemia may contribute to the development of impaired cognitive function, via neuroinflammation, in an animal model of cardiac POCD.

Using in-vivo coronary artery ischaemia, following surgical thoracotomy in rats, the authors studied whether surgery or surgery along with myocardial ischaemia affected the induction of long-term potentiation (LTP), an electrophysiological surrogate of memory formation, in the hippocampus in vivo. The authors also measured mRNA levels of inflammatory cytokines in the hippocampus and correlated the observed changes in LTP with increased neuroinflammation. Their conclusion that myocardial ischaemia may be associated with the development of impaired cognition appears consistent with clinical studies showing association between atherosclerosis and cognitive decline in humans [8]. Of course, in the animal model, rats do not have comorbidities prior to the insult unlike the typical patients who sustain myocardial ischaemia. Nonetheless, this study does indicate that acute organ ischaemia can induce inflammatory changes in the brain that may contribute to impaired hippocampal function. Although this is a fascinating finding, this first report remains far removed from clinical practice. Firstly, as the authors concede, the use of LTP is not equivalent to tests of cognitive dysfunction, and this requires direct testing after induced myocardial ischaemia in animals. Secondly, although systemic haemodynamics were unaltered by the insult, direct assessment of cerebral perfusion was not assessed, and, therefore, the changes in LTP could still represent a direct cerebral effect [9]. Thirdly, although plausible [6], causal association between the neuroinflammatory response and changes in LTP was not definitively proven.

Myocardial injury and neuroinflammation

The possibility that myocardial ischaemia could induce cognitive impairments is intriguing, if not entirely novel. Induced renal (but not liver) ischaemia has been shown to produce neuromotor impairment in mice [10]; interestingly, this correlated with neuroinflammatory changes similar to the observations presented by Zhu et al.[7] and in other animal models of POCD [6]. Although Zhu et al. [7] focus on the impact that myocardial ischaemia may confer to POCD, the importance of any association between myocardial injury and cognitive function may go beyond the boundaries of the operating room to affect patients undergoing acute coronary syndromes in all environments. Therefore, the hypothesis that myocardial ischaemia is associated with the production of cognitive deficits warrants further investigation, especially given the support from cohort studies [8].

Remote organ injury

The concept of an injury to one organ producing an injury in another is not new; however, little is known about the mechanisms involved. Studying how renal injury induces pulmonary [11] or brain dysfunction [10] or how liver injury induces renal failure [12] will likely identify novel therapeutic targets that may be modifiable to prevent remote organ injury. The same rationale could be applied to the development of therapeutics to stop the elaboration of multiple organ failure in traumatic, burn or septic injury. Multiple possible mechanisms may be involved in producing remote organ injury, although accumulating evidence suggests a prominent role for the immune system [10–12]. Indeed, acute kidney injury upregulates proinflammatory and proapoptotic markers in the lung [11]; likewise, liver ischaemia reperfusion injury induces similar changes in the kidney [12], adding to the evidence that myocardial ischaemia induces inflammatory changes in the brain [7]. Whether enough can be learned of the immunological and other possible effectors involved to prevent remote organ injury remains to be seen.

Postoperative cognitive dysfunction and surgery

Interestingly, in the model by Zhu et al.[7], the surgical insult, thoracotomy, did not induce a neuroinflammatory response or changes in LTP. This appears to contradict our recent work demonstrating that splenectomy [6] or tibial surgery [13] induces neuroinflammation with direct effects on animal cognition. Multiple reasons may explain this discordance; species differences and the sensitivity of the cytokine assays used may explain differences in the inflammatory markers, and while we used cognitive dysfunction, the authors used LTP as a surrogate marker of memory formation. It is certainly possible that effects on LTP in one brain region insufficiently model the more global cognitive effects on these animals, only identifying severe insults. Despite these differences, this work adds further credence to the working hypothesis that neuroinflammation is causally associated with the development of POCD. We have now demonstrated using two surgical models that animal POCD is associated with exposure to surgery and anaesthesia, and that this correlates with changes in inflammatory cytokines, including changes in interleukin (IL)-1β; furthermore, POCD is reduced with treatment with an antagonist of the IL-1 receptor [6,13]. As systemic IL-1β (among other cytokines) is measurable after myocardial infarction [14], systemic IL-1β may underlie the cognitive dysfunction observable in both Zhu et al. [7] and our model.

Could anaesthesia be protective against postoperative cognitive dysfunction?

Zhu et al. [7] also demonstrated that sevoflurane is able to precondition against both the inflammatory changes and impairment of LTP. Although it is unclear how sevoflurane achieved this benefit and whether this involved cardiac, neuronal or immunological targets, the anti-inflammatory effects of volatile anaesthetics are increasingly recognized [15]. Sevoflurane has been shown to be able to precondition a wide variety of organs, including cardiac [16], liver [17], brain [18] and kidney [19], against various insults, with the suppression of inflammatory responses commonly cited as a potential mechanism of protection [15]. However, it should be noted that sevoflurane also appears to protect against direct cellular injury [18]. Translation to the clinical arena may yet be premature; the present data from cardiac surgery do not indicate that widespread adoption of preconditioning therapy for organ protection is warranted. De Hert et al. [20] demonstrated that administration of sevoflurane throughout cardiac surgery is more effective at cardiac protection than merely preconditioning with sevoflurane prior to the case [with anaesthetic maintenance with total intravenous anaesthesia (TIVA)]. Therefore, it appears that, to gain maximum benefit from the organ-protective properties of volatile anaesthetics, they should be administered throughout the insult and not just prior to the insult. Whether there is a case for continuing administration of volatile anaesthetics postoperatively for high-risk surgery, similar to prolonged hypothermic therapy [21], requires investigation.

We must balance these potential benefits from the organ-protective and anti-inflammatory effects of volatile anaesthetics against any contribution to POCD. Although data so far suggest that anaesthetics do not appear to induce cognitive dysfunction in our model [6,13], volatile anaesthetic-induced cognitive impairment has been noted in young [22] and adult animals [23], though enhanced cognitive function [24] has also been reported. Therefore, in preclinical models, the contribution of volatile anaesthesia to POCD is unclear. Unfortunately, a clinical study into the effects of general versus regional anaesthesia in POCD was inconclusive due to premature cessation of the trial due to increased mortality in the general anaesthesia group, though a trend to improved cognition was noted in the regional anaesthesia group at 1 week but not at 3 months [25]. Systematic review has been unable to discover any association between general anaesthesia and POCD [26]; that is not to say that it does not exist, but any link is yet to be identified. Could an anaesthetic be therapeutic for POCD? Possibly, an anaesthetic that inhibits various forms of injury may be therapeutic. Sevoflurane [15–20] and xenon [18] are good examples of organ-protective anaesthetics that may be useful in this context. Adjunctive therapies such as minocycline [13] or dexmedetomidine [27] may also have a role, but none of these agents have an evidence base indicating widespread clinical adoption.

Conclusion

The study in this issue of the European Journal of Anaesthesiology by Zhu et al. [7] is fascinating and has highlighted the potential for myocardial ischaemia to induce changes in cognition. If supported by further, in depth, preclinical and clinical work, this finding may have an impact across the spectrum of medical and surgical patients, from surgical intensive care to chronic cardiac therapies in the community. Furthermore, the authors have leant further credence to our working hypothesis that POCD has a significant inflammatory component; we hope that, by targeting the effectors in this process, we may identify therapeutics to inhibit the formation of POCD. Finally, the authors have shown the potential of sevoflurane preconditioning to combat this injury. Although the application of volatile anaesthetics for organ protection, including as a therapy for POCD in high-risk surgery, still requires evaluation, the balance between the protective and toxic qualities of different anaesthetics remains ill defined.

Acknowledgements

Professor Maze has acted as a paid consultant for Air Products, Allentown, Pennsylvania, and both Professor Maze and Dr Sanders have acted in this capacity for Air Liquide Sante International, Paris, France. In addition, Dr Sanders has received an unrestricted travel grant from BOC Ltd, Guildford, UK, to attend the World Congress in Anaesthesia. Air Products and BOC Ltd have funded and continue to fund work in these authors' laboratories. Professor Maze has been a consultant for Abbott Laboratories, Abbott Park, Illinois, USA, to facilitate registration of dexmedetomidine in the United States.

Support was provided solely from institutional and/or departmental sources.

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